Negad El-Borai told AFP he was stopped at Cairo airport, from where he intended to travel to Jordan, and told he could not leave.
"They didn't tell me the reason," he said.
El-Borai said he was under investigation for allegedly setting up an "illegal" law firm involved in drafting an anti-torture bill and in a controversial probe into foreign funding of civil society groups.
A security official at Cairo airport confirmed El-Borai had been prevented from travelling.
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The United Nations said it was "extremely concerned" by the decision to freeze the assets of the activists and NGOs, which stoked fears of an intensified crackdown on civil society.
The investigation, which began in 2011, caused a diplomatic rift between Washington and Cairo after police closed down several foreign NGOs and sought to arrest their American staff.
In 2014, a court sentenced 43 Egyptians and foreigners in the case to up to five years in prison.
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